The data in the OPF file should not affect whether the Add Basic button works or not. Its conceivable that the data in the OPF prevents the Metadata Editor from opening if it were corrupt. But you indicated it opens. I don't see how the Add Basic button is not popping up a list for you (if it isn't then this is quite interesting). Remember - clicking Add Basic is so you can select a new entry - it does not show your existing entries. Your existing entries - if they are EPUB 2 standard - will show up as soon as you open the Metadata Editor.

If the real issue is that the Metadata Editor is not showing your entries, its because they are using a non-EPUB 2 standard format - a format specific to calibre. You can manually enter entries in the Metadata Editor - they'll be put alongside your existing ones in the OPF.